M. L. W. Thewalt

TYPE II PHOTOLUMINESCENCE AND CONDUCTION BAND, OFFSETS OF GAASSB/INGAAS AND GAASSB/INP HETEROSTRUCTURES GROWN BY METALORGANIC VAPOR PHASE EPITAXY

The optical properties of lattice-matched GaAsSb/InGaAs/InP heterostructures with a varying InGaAs layer thickness (0–900 A) were investigated. These structures display strong low temperature type II luminescence, the energy of which varies with the InGaAs layer thickness and ranges from 0.453 to 0.63 eV. The type II luminescence was used to determine directly and accurately the conduction band offset of these structures. The values obtained herein are 0.36 and 0.18 eV at 4.2 K for the GaAsSb/InGaAs and GaAsSb/InP heterojunctions, respectively, with the GaAsSb conduction band higher in energy.

Defect‐free band‐edge photoluminescence and band gap measurement of pseudomorphic Si1−x−yGexCy alloy layers on Si (100)

Pseudomorphic Si1−x−yGexCy alloy layers on Si (100) with band‐edge photoluminescence and without defect‐related luminescence have been achieved. The photoluminescence was measured from 2 to 77 K and was used to make a direct measurement of the band gap shift as a function of strain reduction as C was added. Compared to the effect of just reducing Ge content, results show that as C is added, strain is reduced more efficiently than the band gap is increased. Furthermore, results imply that a fully strain‐compensated Si1−x−yGexCy layer on Si (100) would have a band gap much less than that of Si, and suggest that initial C incorporation reduces the band gap of relaxed, unstrained Si1−x−yGexCy alloys.

Reaching the quantum limit of sensitivity in electron spin resonance

The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology and materials science, from in vivo imaging to distance measurements in spin-labelled proteins. ESR relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession. However, such signals can be very small, prohibiting the application of ESR at the nanoscale (for example, at the single-cell level or on individual nanoparticles). Here, using a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly four orders of magnitude. We demonstrate the detection of 1,700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise ratio, reduced to 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼ 0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new versatile toolbox for ESR at the nanoscale.

Exciton luminescence in Si1−xGex/Si heterostructures grown by molecular beam epitaxy

Coherent Si1−xGex alloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low‐temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon‐resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. The broad PL band was predominant when the alloy layer thickness was greater than 40–100 A, depending on x and the strain energy density. The strength of the broad PL band was correlated with the areal density (up to ∼109 cm−2) of strain perturbations (local lattice dilation ∼15 A in diameter) observed in plan‐view TEM. Thinner alloy layers exhibited phonon‐resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescence excitation spectroscopy, external quantum efficiency, time‐resolved PL decay, together with the power and temperature dependence of luminescence inten...

Low‐temperature photoluminescence of epitaxial InAs

Photoluminescence studies as well as reflectance and transmittance measurements were performed on high‐purity epitaxial InAs grown by metal‐organic chemical‐vapor deposition. We report the optical identification of excitonic, donor, and acceptor impurity related transitions at a temperature of 1.4 K. Measurements at higher temperature and in the presence of magnetic fields up to 7 T support these identifications. We find the excitonic band gap at 415.65±0.01 meV according to the minimum in the polariton reflectance feature. The donor–acceptor‐pair and acceptor‐bound exciton transitions for three different acceptors are observed by photoluminescence, and we tentatively associate one of them to a double acceptor formed by a Ga impurity on an As lattice site. A donor‐bound exciton transition is observed with a binding energy of 0.42 meV. The magnetic field dependence yields values of the electron effective mass and g factor of (0.026±0.002)m0 and −15.3±0.2, respectively, in good agreement with values obtaine...

Strain balanced InAs/InAsSb superlattice structures with optical emission to 10 μm

We report the growth and optical characterization of InAsSb/InAs strain balanced superlattice structures on GaSb substrates for potential application in midinfrared photodetectors. Photoluminescence (PL) emission was observed in the range 5 μm≤λ≤10 μm at 4 K for Sb compositions 0.14≤xSb≤0.27. The PL energy was found to depend approximately linearly on antimony, consistent with a type II band lineup. The dependence of the emission energies on the Sb mole fraction is in agreement with trends predicted by various theoretical works. The data suggest that this transition reaches zero energy for a composition of xSb=0.37.

InAs/InAsSb strain balanced superlattices for optical detectors: Material properties and energy band simulations

InAsSb/InAs type II strain balanced superlattices lattice matched to GaSb have recently been proposed as an alternative to InAs/(In)GaSb short period superlattices for mid- to long infrared photodetectors. Photoluminescence data at 4 K of OMVPE grown InAsSb (multi-) quantum wells in an InAs matrix on InAs and GaSb substrates is presented for Sb compositions between 4% and 27%. The measured transition energies are simulated with a self-consistent Poisson and Schroedinger equation solver that includes strain and band-offsets. The fitted parameters are then used to predict the type II transition energies of InAsSb/InAs strain balanced superlattice absorber stacks at 77 K for different compositions and periods. The optical matrix element was calculated and compared with InAs/(In)GaSb superlattices. The InAsSb/InAs structures can be designed with higher or equal matrix elements for longer periods. Finally, the initial optical response data of an unoptimized strain balanced InAs0.79Sb0.21/InAs detector with a 4...

Growth and band gap of strained 〈110〉 Si1−xGex layers on silicon substrates by chemical vapor deposition

We report chemical vapor deposition growth of strained Si1−xGex alloy layers on 〈110〉 Si substrates. Compared to the same growth conditions on 〈100〉 substrates, a slightly lower Ge composition and a much lower growth rate was observed. From photoluminescence measurements, the band gap of these films for 0.16≤x≤0.43 is evaluated and compared to theory. Finally, a surprisingly large ‘‘no‐phonon’’ replica line strength ratio was observed as compared with that observed in 〈100〉 layers.

Modification of the shapes of GaAs/AlGaAs quantum wells using rapid thermal annealing

Abstract The effect of rapid thermal annealing (RTA) on the shapes of single and coupled double GaAs/AlGaAs quantum wells has been investigated by measuring exciton energies using low temperature photoluminescence and photoluminescence excitation spectroscopies. After RTA, large changes in exciton energies were observed only if the sample was capped with a thin layer of silicon dioxide. For single quantum wells there was an increase in exciton energy whose magnitude depended on the width of the well and its distance from the surface of the wafer. For coupled quantum wells, the exciton energy decreased and there was clear evidence, in the excitation spectrum, of asymmetry in the heterostructure after RTA.

Direct observation of the donor nuclear spin in a near-gap bound exciton transition: P31 in highly enriched S28ia)

We report on ultrahigh resolution studies of the bound exciton states associated with the shallow acceptor B and the shallow donor P in highly enriched S28i using a tuneable single frequency laser to perform photoluminescence excitation spectroscopy. The linewidths and fine structure of the transitions, which were too narrow to be resolved previously using an available photoluminescence apparatus, are now fully revealed. The P bound exciton transition shows a complicated additional structure, which the Zeeman spectroscopy demonstrates to be a result of the splitting of the donor ground state by the hyperfine interaction between the spin of the donor electron and that of the P31 nucleus. The P31 nuclear spin populations can thus be determined, and hopefully modified, by optical means. The predominant Auger recombination channel of these bound excitons is used to observe the same resolved hyperfine transitions in the photocurrent spectrum. This demonstrates that donors in specific electronic and nuclear spi...